Myelination and demyelination are important areas of research in developmental neurobiology. Myelination of nerve fibers is essential for the function of the vertebrate nervous system. The myelin sheath covers an axon along its entire length with periodically ocurring gaps at the nodes of Ranviers. This distribution of long insulated stretches (the internodes) and short conducting regions (the nodes of Ranvier) is the basis for the rapid conduction of nerve impulses characteristic for the vertebrate nervous system. The insulating myelin sheath is elaborated by highly specialized glial cells, Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS). In the absence of this .sheath, as seen in demyelinating diseases, impulse conduction is impeded resulting in severe sensory and motor deficits (1). The myelin structure has been investigated for decades, first focussed on morphological aspects, later on the biochemical and molecular composition. In the PNS, myelin is synthesized by Schwann cells, each of which elaborates a segment of myelin (or internode) around a single axon. In the CNS one myelinating oligodendrocyte can form up to 40 myelin internodes around different axons. This mode of interaction decreases the demand for myelinating glial cells and is understood as an adaptation to the limited space within the complex vertebrate brain (2). Myelination in the CNS starts around birth with the differentiation of oligodendrocytes which grow out processes to contact the nerve fibers. These processes form membranous
sheets that wrap spirally around the axons many times. With the compaction of the spirals the cytoplasm is displaced to the edges of the membrane sheets. he such generated stacks of membranes, in which the cy. toplasmic appositions according to their electron density have been designated ´the major dense line´ and the extracellular appositions ´the intraperiod lire´, form excellent electric insulators (Fig.1).
In addition to specific glycolipids like e.g. galac. tocerebroside (GaIC) and sulfatide, oligodendrocytes produce a number of proteins which are exclusively found in myelin. The two major constituents of myelin, which together make up 80% of the protein content, are myelin basic protein (MBP) and proteolipid protein (PLP). Both proteins are important structural components of myelin that are involved in the compaction of the myelin sheath. Minor constituents in elude the glycoproteins myelin-associated glycoprotein (MAG), oligodendrocyte-myelin glycoprotein (OMgp) and myelin/oligodendrocyte glycoprotein (MOG), of which MAG is characterized best (3,4,5,6)..
Here the molecular biology of the myelin proteins is presented first and then the factors involved in the generation of oligodendrocytes are described.
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